A large number of techniques have been developed to improve the spectral resolution in Nuclear Magnetic Resonance (NMR) of solid samples, and most modern techniques include extremely rapid spinning (10–30 kHz) of the sample at the “Magic Angle” (the zero of the second Legendre polynomial, 54.7°) with respect to B0. If the rotational rate is fast compared to chemical shift anisotropies and dipolar couplings (in units of Hz), the resolution is dramatically improved—often by two or three orders of magnitude. Even when the spinning is not fast enough to satisfy the above conditions, substantial improvements in resolution are generally obtained from the combination of MAS and multiple-pulse methods. MAS NMR spinners of several types have been disclosed, for example, in U.S. Pat. Nos. 4,275,350, 4,446,430, 4,456,882, 4,739,270, 5,202,633 and 6,803,764.
The MAS spinner is but one component of the NMR MAS probe or probe head, which is inserted into the bore of a superconducting high-field, high-homogeneity magnet. The other primary components of the probe include the rf tuning circuitry, an example of which is disclosed in U.S. Pat. No. 6,130,537, and temperature control means, according to the prior art.
This invention pertains specifically to a novel method of spin-rate detection. Three methods have been widely used for the past three decades—acoustic, triboelectric, and optical. Barbara, in U.S. Pat. No. 5,170,120 discloses an MAS technique that depends critically on synchronous pulsing and hence spin-rate detection with good phase accuracy and stability. Many other MAS techniques also require pulsing synchronously with the spinning; and for this reason, most modern MAS NMR probes rely on optical spin rate detection. Under proper conditions, they may achieve rotational phase accuracy of a few degrees over a range of rotational rates from tens of Hertz to over 20 kHz.
One of the earliest spin-rate detection methods involved mounting an electret-based, non-magnetic microphone about 30 cm from the spinner and amplifying and filtering the signal. The problem with this method is that the acoustic signal strength at the fundamental frequency from a well-balanced rotor is generally too low for adequate phase stability of the detection, even with sophisticated filtering.
The conventional triboelectric method relies on the asymmetrical build up of surface charges on the ceramic rotor surface from the high-velocity bearing gases flowing over it. This random and hence asymmetrical spinning surface charge produces a modulated electric field that may be detected by a small, high-impedance, dipole antenna. Of course, relying on a random process makes phase accuracy impossible, and the signal strength often drops below the noise level. Also, the harmonic content of the signal is unpredictable. Still, this method is often used, as it is simple to implement, while the optical method is not always as easy to implement as one might suppose.
The challenges of implementing optical spin rate detection arise predominately from the fact that space is extremely precious near the spinning sample, as every bit is needed to optimize both the rf coils and the spinner system. Moreover, the light pipes must be quite small if they come very close to the sample region to avoid unacceptable perturbation of the magnetic field homogeneity, either as a result of their own susceptibility or as a result of requiring a window in the spinner stator, which is generally diamagnetic. Small light pipes are fragile, and it has often been difficult to achieve adequate S/N in the optical signal for the desired phase accuracy.
The challenges of implementing optical spin rate detection appear to increase by an order of magnitude in the case of a CryoMAS™ probe, where a ceramic dewar is required between the sample rotor and the rf coils. In other cases too, including Switched Angle Spinning (SAS), as discussed in more detail in U.S. Pat. No. 6,198,284, and in spinners compatible with automatic sample change in narrow bore magnets, as discussed in more detail in co-pending application Ser. No. 11/163,344 filed Oct. 14, 2005 on MAS Inflow Bernoulli Bearings, now published as US publication number [to be provided upon publication], which co-pending application is hereby incorporated herein by reference for all purposes, a more practical method of spin rate detection with high phase stability is needed.
U.S. Pat. No. 6,803,764 claims the use of a gold film, deposited on the rotor, for reflection of the optical spin-rate detection signal, where the gold film is sufficiently thin to prevent it from coming off from the high centrifugal forces. It is not clear that the gold film can be made thin enough to prevent eddy current problems in very high fields and still be thick enough to be a good reflector of light. It is instructive to note that turbines with number of blades prime to the number of nozzles, as cited in the independent claim in this patent, were in the Doty Scientific 4 mm production model XC4 in 1998, and those 4 mm rotors routinely spin at 25 kHz.
The instant invention utilizes a material within the spinner (herein referring to the combination of the ceramic rotor tube, its end caps, and the sample) having a permanent net electric field. Such materials include ferroelectret films, in which the electric polarization generally arises from opposite mono-polar charges on separated surfaces within a structured material containing voids, or from piezoelectric materials, also known as a ferroelectric materials (the name comes from analogy to permanent magnets), in which the electric polarization arises from a crystallize structure lacking an inversion center.
Rotation of electrically polarized material produces a time-dependent electric field, modulated at the rotational frequency, so that means similar to the conventional triboelectric method achieve the needed high sensitivity and phase stability. If the electrically polarized material is also of sufficiently high resistivity, modulation of the external NMR polarizing magnetic field B0 will be negligible, as desired for NMR applications.
Piezoelectric materials have been widely utilized for at least six decades in microphones, and related electret film materials have been widely utilized for nearly four decades in microphones and other transducer applications. Over the past decade, there has been substantial progress in the production of porous polymeric ferroelectret film materials that have seen a wide range of sensor applications. See, for example, the publicly available paper “Electret Sensors, Filters, and MEMS Devices” by Malti Goel, in Current Science, Vol. 85, #4, 25th Aug., 2003, pp 443–453, or the information available at the web site of the company EMFIT, Ltd. A recent advance in the production of PTFE electret films is disclosed in U.S. Pat. No. 6,818,092. Generally, electret sensors require electrodes for connecting to an external circuit. An important characteristic of the instant invention is that the electrodes are preferably omitted, or a least of very high resistivity, so as to avoid deleterious eddy currents when rotating in a high external magnetic field.